1. Field of the Invention
The present invention concerns a method and an x-ray apparatus for radiation exposure of a patient who can be placed at a variable distance relative to an x-ray source.
2. Description of the Prior Art
X-ray methods and x-ray apparatuses have been used for many decades for medical imaging, in which x-rays are directed through the body of a patient in the form of a living person or animal. Despite this large usage in medicine, such ionizing radiation is not free of health risks for the patient. The use of x-ray radiation on living patients is therefore heavily legally regulated. In particular, maximal allowable limit values exist for the dose rate of x-ray radiation on the body surface of a patient. 21 C.F.R. (Code of Federal Regulations) of the Department of Health and Human Services is an example. 21 C.F.R. §1020.32e specifies what is known as the maximum allowed input exposure rate, thus the entrance skin dose rate in mGy/s on the side of a patient facing towards the radiator.
The dose rate radiated by an x-ray source onto a given monitoring surface decreases quadratically with the distance of the monitoring surface from the x-ray source. A minimum distance that must be maintained between a biological tissue and the focus of an x-ray source or the x-ray tube in order to not exceed the prescribed limit dose rate (thus the entrance exposure rate) is calculated from the quadratic distance rule and a predetermined maximum value (limit value).
This minimum distance is primarily dependent on the primary dose rate radiated by the x-ray source and thus is dependent on the limit energy of the x-ray radiation as well as on the maximum achievable beam current.
In specific x-ray applications, the distance between the x-ray radiation for us and the patient is always fixed. The x-ray radiation is then set (by the design of the x-ray tube or by the voltage/current generator or by side filters placed in the beam path) such that no patient is irradiated with an unacceptably high dose rate.
The behavior is different in x-ray systems of the type used, for example, in urology. In arrangements known as under-table fluoroscopy workstations, the x-ray radiator is located under a patient table. The patient table is adjustable in terms of height, meaning that the distance between a patient reclining on the table, or the table surface and the x-ray radiator or its focus is variable. Table positions of different heights are desirable so that a urologist working at the table or on the patient located thereon can move the table into an ergonomically-advantageous working position.
For the most part, the patient table can be moved toward the x-ray radiator so far that, upon operation of the x-ray radiator, the patient lying on the table would be irradiated with an unacceptably high dose rate. If the urologist is standing, the table is generally upwardly by a relatively large distance, causing the patient to be positioned outside of the critical dose rate. An x-ray fluoroscopy of the patient is then possible without danger. For longer endo-urological procedures under x-ray monitoring, however, the urologist usually prefers to be seated, so the patient is moved downwardly, thus very close to the x-ray radiator. The minimum distance may then no longer be adhered to, which (given an activated x-ray source) would lead to an unacceptably high entrance dose rate at the patient.
Various technical possibilities are known in order to always prevent exposure of the patient with a dose rate that is too high. In the designer installation of an x-ray source, fixed primary aluminum filters can be installed in the x-ray radiator in the beam path of the x-ray system between the x-ray source and the patient in order to decrease the maximal primary dose rate that can be emitted by the x-ray source and that can actually strike the patient. The safety distance between the x-ray source and the patient, thus the minimum allowable distance, is thus smaller than without a primary aluminum filter, so that (for example) a urological work table can be lowered further. The radiation, however, is hardened by the primary aluminum filter, causing the image noise in the x-ray image to increase in a disruptive manner, and in fact at distances between the patient and the x-ray source that would be non-critical given unattenuated x-ray radiation (without an aluminum filter). Metal filters (also made from copper) filter the soft radiation portion from the x-ray radiation; so the generated image is thus also higher-contrast, but the image brightness remains.
Furthermore, it is known to mechanically limit the movement path of the patient table in a fixed manner such that thus cannot be moved closer to the x-ray source than is prescribed by the safety interval. This limits the ergonomics of the arrangement since the table is precluded from being lowered when the urologist would like to execute a procedure without x-ray exposure.
It is also known to deactivate the x-ray source or to block triggering thereof if and when the safety interval is not adhered to. In such a table position, however, x-ray fluoroscopy of the patient is no longer possible at all. In such a situation, the doctor must refrain from moving the x-ray table into a position that is ergonomically advantageous for him or her, or must move it upwardly from a convenient working position in order to obtain a fluoroscopy of the patient.
Furthermore, it is known to permanently operate the x-ray radiator below its maximum dose rate, thus with a lower product made up of beam current and acceleration energy than the x-ray source could actually maximally emit. This likewise leads to a permanent decrease of the maximum safety distance. As in the case of a permanently-installed filter, however, the image quality of the x-ray system is generally degraded since the x-ray source is never operated at its optimal operating point. This also applies for table positions that are actually situated above the safety distance and would otherwise allow a higher image quality. The decrease of the generator power leads to a decrease of the number of generated x-ray quanta and an increase of the quantum noise. The x-ray image is coarser, appears noisy and is no longer regarded as being acceptable.
All known solutions are thus compromises between the x-ray contrast or the x-ray image quality and the ergonomics of the overall arrangement.